thank you sir.......

As long ago as 40 yrs, I remember that the firm where I worked (DuPont) utilized butterfly valves in dilute phase pneumatic transfer systems for modulation control.

Presumably they are rather non-linear. But they have worked in carburettors just fine for many years!

Del
(the last post before I'm off for a few days ...wait for the withdrawl symptoms to kick in as I go over the Thames on the QEII Bridge )

<.....carburettors......>

Good example.

carburettor throttle damper can not be compared to butterfly valve modulation pattern.

when it is at idle speed it uses different path for air...

butterfly's are poorly handling low flow condition and of course cavitation is serious issue.

However there are systems where butterfly valves were used for flow modulation but they had been configured so that two different size valves operate in parallel and during low flow the large one is shut and smaller one modulates.... high maintenance.

Spend money and get proper size globe valve. Unless you have free labourers to do repairs.

G.M.

The two uses for butterflys - liquid and gas - give very different answers so far as practicality goes. In gas control, butterflys work very well; I've used dozens, both automatic and manual. In fact, I'm using several right now. In liquid control, some of the issues you raise do come up.

Do you remember when chokes had a manual control for any opening you desired (infinitely variable)?

I agree, all depends on application. There is use for everything...

Stephan is right, operating parameters are to be considered and then equipment selected.

I don't know who told you that, but they are wrong. Butterfly valves are very non-linear (so is a ball valve) and might not work well in a cheap and dirty setup but they are widely used in servovalves where the loop is tight enough to make non-linearities insignificant. Just Google servovalves (or even control valves) and butterfly for a list of companies making these.

And, there are setups where you don't actually care about non-linear performance, i.e., where the frequency response is quite low (for example the one on my chainsaw which takes about 8 minutes to respond to a new choke setting!).

You can, actually, use butterfly valves for continuous control. We have several of them in our factory, as a matter of fact.

This is not to say that you're allowed to use them anywhere. The only reason we use them is because they're used to control air flow in large ducts. It would be pretty cumbersome to use 48" globe valves .

Having said that, we're thinking of eliminating the butterfly valves (or dampers, as we call them) by controlling the speed of the fans that blow the air through the ducts. There's no need for calibration and they save power too.

They can't modulate like a CPM (Constant Pressure Modulate) valve can.

for varying flow rates, whilr the butterfly is difficult to vary the flow rates.

phoenix911

Try to simulate its use manually and you will get the answer.

I tried to control the flow of air generated by centrifugal fan with the help of butterfly valve. The 90 degrees of butterfly valve were divided into 9 parts. each part had the locking position. Initially butterfly valve was closed. When I opened 10 degrees, I found that nearly 22 % of the air passed through the valve. When I opened the valve 20 degrees, nearly 40% air passed through valve. When I opened the valve by 30 degrees, nearly 54% air passed through valve. This means you have to calibrate the 4-20 mA range with non linear curve. the curve will be different for every fluid passing through valve if its resistance to flow is different. Linear calibration of control will lead you to incorrect opening then the actual requirement.

Butterfly valves are very commonly used in vacuum applications to maintain a certain pressure in a chamber by throttling the flow to the vacuum pump. I find that the biggest problem with these is that the butterfly itself usually has a o-ring seal around its edge which wears out over time (depending on how it cycles).

Avery Montembeault

You just basically described the carburetor example.

The answer is that you can use a butterfly in cases where you want a non-linear control valve and don't use it when you want a very linear control over the range of % open for the valve.

Look at the valve as if you were standing inside the pipe. In your mind (or your fancy 3D modelling software if you have it) slowly close the valve in 10% increments as mentioned above. Measure the flow area change and I think you will understand it.

Stephan

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www.bradleyventures.com

Butterfly valves used to be very commonly used as throttle valves in vacuum systems in the semiconductor industry. They were found to be the cause of lots of particle problems so have become much less common.

The deal is for air flow (without temperatures and pressures that would trigger phase change) a butterfly valve seems adequate. Yet for liquids, the differential (between the low and high pressure) at the same temperature will potentially introduce the cavitation problems described above (a costly head ache).

Consider your application details (including the flow rates and volume) to better understand you options.

Good luck!

Hello

Butterfly valves can be used, but they are for sure not the best choice. Their opening curve is unpractical. You only have to open a little bit to get a lot of flow because the opening is on two sides. So after opening about 45°(50% of 90°) the flow is much more than 50%. I have used butterfly valves on vessels for cooling water control and sea water control, and they can only be used for rough adjustments.

Kv. Gudm J.A. Iceland

For linearity nothing beats the plug valve. Of course really large sizes aren't all that practical.

why do we care about linearity any more with positioners and electronic self tuning PID loops?

Because in a linear valve, it takes the same amount of position change to make the same amount of flow change anywhere else on the curve. With a non-linear valve, the position change varies at all points on the curve.

For example:

In a non-linear valve, say you want to change from 5 gpm to 10 gpm. This takes, say, a 10% change in position. If you want to change from 10 gpm to 15 gpm, it may take a 15% change in position. This means it takes longer for the valve to achieve the same amount of change at different flows.

Usually, a valve positioner has a limit in the speed that it can move. Even if you give a command to move faster, once the valve has reached its speed limit, it won't move any faster. With a linear valve, any limitations will be spread out all along the curve.

Further, electro-pneumatic valve positioners cannot respond to a change of 2% or less and the pneumatics are not 100% accurate or precise. The newer digital positioners are only slightly better. So, if you need to a small change at one point in the curve, your positioner might not respond.

Butterfly valves are commonly used in automatic control, but the optimum is the globe valve, where its circular disc and seat help the fluid to be equally and smoothly distributed around the disc, with a little friction loss. Also, when such deterioration happened to the disc and/or seat, it is found symmetrically along the peripherals which allow good tighten in OFF case.

good explanation Abdel Halim Galala,

Also having a butterfly valve, when the flows drop. I experienced hammer, and using it, as a flow control this would make it difficult for that same reason you described, true?

phoenix911

Why do we care about friction loss isn't that what the valve is suppose to do?

I do like your explaination about seat wear versus ware on a the butterfly seal. However, the butterfly is used for low pressure dop applications and seat wear on them is minimal.

Why do we care about friction loss isn't that what the valve is suppose to do?

The Valves are designed and suppose to do one (or more) of the following 5 functions:

a. On/Off : Stopping or starting flow.

b. Regulating :Varying the rate of flow.

c. Checking : Permitting flow in one direction only.

d. Switching :Switching flow along different routes.

e. Discharging :Discharging fluid from a system.

And we prefer to use the valve which has a lower friction loss, in all cases we have to take that friction loss (drop of pressure) into consideration.

I belive also we can consider the valve as a main item used in the design to creat a pressure drop in the system. ( Pressure Control Valves)

I would suggest that your application would dictate what type of valve you use to control flow across the valve, hence downstream. If the material is a solid, I wouldn't say you can't use a butterfly or knife gate valve to control volume. If you are moving a liquid, a much different story. The downside to using a butterfly valve as a "continuous flow control" is the dynamics going on across the "flapper" when you bring the valve off fully closed or fully open. If you intend to "choke" to gain volume control, a knife gate is much better suited, as it is actuated and controlled by either a pneumatic or hydraulic cylinder, which gives very fine control over the open orifice. If it is a liquid, there are other dynamics you have to consider, and either a flow control or proportional valve is much better suited. The why is not as important as the selection of the proper valve for the application. That is why there are quite a few of us that specialize in applications Engineering.

Because butterfly valves typically have the narrowest control range, they are generally best suited for fixed-load applications. In addition, they must be carefully sized for optimal performance at fixed loads. They provide high capacity with

low pressure loss through the valves. Butterfly valve bodies offer economy,

particularly in larger sizes and in terms of flow capacity per investment dollar.

Butterfly valve bodies might require high-output or large actuators if the valve is big or the pressure drop is high, because operating torques might be quite large..

Standard butterfly valves are available in sizes through 72-inch for miscellaneous control valve applications. Smaller sizes can use versions of traditional diaphragm or piston pneumatic actuators, including the modern rotary actuator styles. Larger

sizes might require high-output electric or long-stroke pneumatic cylinder actuators. Butterfly valves exhibit an approximately equal percentage flow characteristic. They can be used for throttling service or for on-off control. Soft-seat construction can be obtained by using a liner or by including an adjustable soft ring in the body or on the face of the disk .

My Refernce on the above is

CONTROL VALVE HANDBOOK

Fourth Edition From Fisher